Silver-metal oxide materials are used as contacts in a variety of electrical devices, such as relays, because of their high conductivity and resistance to welding that can occur between contacts. The silver provides high conductivity. The metal oxide provides resistance to welding. Currently, the contact material of choice is silver-cadmium oxide because it has the desired conductivity and weld resistance and is easy to make. Silver-cadmium oxide contact materials typically contain about 7 weight percent (wt %) to about 13 wt % oxide.
Recently, concerns about the toxicity of airborne cadmium and cadmium oxide particles have led some jurisdictions to propose regulations to lower the amount of such particles permitted in manufacturing environments. Complying with these regulations will require process changes that will make silver-cadmium oxide materials too expensive for use in many commercial applications. As a result, silver-cadmium oxide is likely to be phased out as a contact material. If that happens, silver-tin oxide is a logical successor. Silver-tin oxide does not present the same toxicity concerns as silver-cadmium oxide and can have superior contact properties. For some applications, silver-tin oxide materials with 7 wt % to 10 wt % oxide are suitable. Many applications, however, require tin oxide contents of at least about 10 wt % to provide adequate weld resistance.
Several methods for making silver-tin oxide contact materials are available. The most straightforward method is to oxidize tin in a silver-tin alloy. When exposed to oxidizing conditions, however, the silver-tin alloy forms an undesirable, tenacious, protective oxide scale that inhibits internal oxidation. As a result, this method cannot make materials with more than about 8 wt % tin oxide. Efforts to overcome this limitation by oxidizing silver-tin alloys in high pressure, pure oxygen atmospheres have been unsuccessful.
Silver-tin oxide materials also can be made by blending and compacting tin oxide powders with silver powders. Materials made with this method can contain more than 10 wt % tin oxide. Often, though, they have flaws that make them unsuitable for electrical contacts. For example, agglomerations of tin oxide particles can create cracks and other physical defects when the material is cold worked to make contacts. The agglomerations form because it is difficult to mix the tin oxide and silver powders uniformly. Efforts to improve mixing by varying the size of the tin oxide powder have been unsuccessful. Another defect found in blended silver-tin oxide materials is due to internal flaws in individual tin oxide particles. These flaws, especially prevalent in particles more than 5 .mu.m in diameter, also create cracks and other physical defects in the silver-tin oxide materials when they are cold worked to make contacts.
At least two other methods of making silver-tin oxide materials are available. In one method, an insoluble tin compound is precipitated from an aqueous solution onto a silver powder. The tin compound is converted to tin oxide and the silver-tin oxide material is consolidated into a suitable form. In the other method, tin and silver compounds are coprecipitated from an aqueous solution. As before, the tin compound is converted to tin oxide and the material is consolidated into an appropriate form. While capable of producing acceptable silver-tin oxide materials, both methods are costly and difficult to adapt for commercial scale production.
Therefore, what is needed in the industry is a method of making silver-metal oxide contact materials that contain adequate amounts of oxide and can be made into electrical contacts.